There is considerable interest in electric vehicles (EVs) as pollution-free transport units, but the public has become so used to the convenience of gasoline as a fuel that the problems of electricity supply and storage still render EVs relatively unacceptable.
Providing power to an EV involves selecting a source. One extreme is where the vehicle carries its own energy on-board--the pure battery approach, with rechargeable batteries charged at a supply depot, and the other extreme is an immediate supply approach without the use of any storage battery at all, perhaps using a pickup brush rubbing on an electrified rail or an overhead wire, or using an inductive power transfer approach. (Some EVs may supplement these methods with solar generated power or occasional use of an on-board generator driven by an engine). Generally it is desirable to provide sufficient charge in a given vehicle to provide for a reasonable distance of travel. It is desirable to provide a convenient, "invisible" charging process so that the user simply gets into the vehicle as and when required, and goes off to a destination. Circumstances determine the most appropriate source selection from this range of choices of supply of power. It may be too expensive to electrify a route. At least some battery storage is preferred for most EVs, so that the vehicle can at least temporarily go off an energised route, and so that peak power levels can be provided at levels which exceed the rate at which power can be transferred from the stationary supply. Some vehicles may return power to the battery during regenerative braking; useful in hilly localities.
EVs supplied from stationary sources of electric power generally remain at a charging site for a period, charge up one or more on-board batteries, and then consume the stored energy during movement. Disadvantages of this process include the extended waiting time at a charging position, and the need to make a deliberate refuelling act from time to time in order to continue to travel. Apart from the disadvantage of relatively frequent refuelling, EVs also have technical problems related to the storage batteries themselves. The lead-acid battery is still by far the most commonly used kind even though novel types such as sodium-based cells have recently been developed--and abandoned--and lithium hydride batteries are now being developed. Lead-acid batteries are marginally acceptable for electric vehicle applications because under existing management regimes they suffer from the disadvantages of high weight, high volume, poor energy storage, poor energy density, poor cycle life, and high cost.
A less well appreciated but significant problem relates to those larger vehicles that employ a number of battery units or monoblocks wired in series. Typically the motors of these vehicles run at 100 or 200 V or more, largely to minimise switching costs and ohmic losses. The deleterious effects of either complete discharge or complete (over) charge of the lead-acid battery are well-known. The problem is further compounded by the inevitably differing coulombmetric efficiencies of a set of battery cells. If a series bank of cells is repeatedly charged then discharged, some cells will tend to drift to a fully charged state while others will tend to drift to a fully discharged state. The prior-art procedure used to equalise the charge in all units of a set is to fully charge the entire set until all batteries starts to `gas`--when every cell in the battery is fully or 100% charged--but most are now overcharged and electrolyte is lost during gassing. For a typical EV, a full charging process is done once per day or overnight, with opportunity charging through the day. It is known that provided a lead-acid battery is maintained at a depth of discharge (DOD) of between about 30% and about 70% of its maximum capacity, it can be exercised through a very much larger number of cycles than if it is charged or discharged to its full capacity.
One difficulty in restricting duty to this 30%-70% range in multiple battery units is that individual units may vary so much in performance that if treated as a homogeneous battery bank over a number of cycles, some units may tend to reach a state of complete charge while others may tend to a state of complete discharge. Consequently the EV exhibits a limited range, and greatly reduced battery life. This problem might be overcome with a total battery care depot where the batteries of an EV are totally removed and replaced by replenished, checked batteries, or by including in each EV a host of individual floating chargers; one for each battery unit or monoblock, together with some means for sensing the state of charge, but providing that kind of charging means has hitherto been regarded as commercially infeasible.
Of course, applications other than vehicles may lend themselves to this type of battery management. For example, a power tool relying on a series of cells could be provided with a battery management unit of this type and so exhibit extended charge capacity and extended battery life. Telephone exchanges for example use banks of massive lead-acid batteries for backup power. The growing use of renewable energy may bring banks of storage batteries into domestic use, for holding excess energy collected from the sun or the wind and returning it for later consumption.
Object
It is an object of the present invention to provide an improved apparatus and method for managing battery charge in a rechargeable battery, or one which will at least provide the public with a useful choice.
Statement of the Invention
In a first broad aspect the invention provides battery charging means for a rechargeable battery unit comprising one or more cells; the battery charging means employing inductive power transfer between at least one primary inductive conductor capable of being energised at a high frequency, and a loosely coupled, resonant, secondary inductive pickup unit capable of supplying a corresponding battery unit with a charging current; said pickup unit including at least one inductor, resonating means, and rectifying means; characterised in that the pickup includes means for varying the coupling between the primary inductive conductor and the inductive pickup unit; the means comprising a shorting switch capable of shorting the at least one inductor from time to time so that the amount of electrical energy circulating in the resonant pickup unit is capable of being controlled and so that the amount of energy passed to the rechargeable battery unit is capable of being controlled.
In a related aspect the invention provides battery charging means as described above, wherein the pickup unit includes a first control inductor and a second power-handling inductor, the first inductor and the second inductor being inductively coupled with each other so that when the first inductor is in a short-circuited condition the second inductor is substantially decoupled from the primary inductive conductor.
Preferably the first inductor is comprised of many turns of litz wire, and the second inductor is comprised of a flat conductive sheet; preferably a flat litz cable or alternatively a solid metallic strip.
In another related aspect the invention provides battery charging means as described above, wherein the pickup unit includes control means for controlling the amount of coupling between the primary inductive conductor and any one pickup winding; the control means being capable of determining the state of charge of the rechargeable battery and, by acting on a short-circuiting switch capable of short-circuiting the first inductor, the control means is capable of providing or not providing further charging current so that the charge in the rechargeable battery is capable of being controlled.
In a further related aspect the invention provides battery charging means as described above, for use with at least one primary inductive conductor and a bank of rechargeable batteries made up of a plurality of battery units; each battery unit being provided with associated battery charging means, wherein the control means is capable of determining the state of charge of each battery unit within the bank and is capable of providing or blocking further charging current to each battery unit so that the state of charge of the corresponding battery unit is controlled and so that the state of charge is equalised throughout the battery units of the bank.
Preferably the state of charge is controlled during charge, storage, and discharge although optionally its activity may be restricted.
In yet another related aspect the invention provides a battery monoblock with battery charging means as described above, wherein the charging and control means is capable of drawing its operating power internally and the charging and control means has a self-stabilising property; whereby the control means is capable of determining whether any single cell holds less charge than any other cell, and the control means is capable of allowing charge to be added to any single under-charged cell so that all cells are brought to the same state of charge, or depth of discharge (DOD).
In a subsidiary aspect the invention provides a monoblock incorporating battery charging means as described above, wherein the charging and control means is physically included within the monoblock.
In a still further related aspect the invention provides a vehicle at least partially powered by at least one electric motor, in which the electric motor draws power from a rechargeable battery bank capable of being charged from battery charging means as described previously.
In a yet still further related aspect the invention provides battery charging means as described above, having an electric power storage module including power collection means capable of collecting energy from a renewable resource, and a rechargeable battery bank capable of storing the collected energy and releasing it to a power conversion device, characterised in that the rechargeable battery bank is capable of being charged and/or stabilised from battery charging and controlling means as previously described.
In a further related aspect the invention provides battery charging means as described above, wherein the controller substantially maintains the charge between predetermined limits appropriate to the type of rechargeable battery in use.
In a more specific aspect the invention provides battery charging means as described above, wherein the rechargeable battery is a lead-acid storage battery and the controller substantially maintains the charge between predetermined limits, being an upper limit of about 70% of full charge, and a predetermined lower limit of about 30% of full charge, so that the lead-acid battery has an enhanced lifetime.
In yet another aspect the invention provides battery charging means as described above, wherein the control means is capable of detecting or anticipating failure within the rechargeable battery bank.
In a subsidiary aspect the invention provides battery charging means as described above, wherein the control means is capable of recording the performance of the or each battery unit.
In a second broad aspect the invention provides battery charging means for a bank of rechargeable batteries; the battery charging means employing inductive power transfer between at least one primary inductive conductor capable of being energised at a high frequency, and a plurality of resonant, secondary pickup windings; each winding being capable of supplying a corresponding unit of the bank of rechargeable batteries with a charging current, wherein there is means for controlling the coupling between the primary inductive conductor and any one pickup winding; the controlling means being capable of determining the state of charge of the corresponding battery unit and of providing or blocking further charging current so that the state of charge of the corresponding battery unit is controlled and so that the state of charge is equalised throughout the units of the bank.
In a related aspect the invention provides battery charging means as described above, wherein the resonant secondary winding provided with controlling means is effectively coupled to another substantially non-resonant secondary winding comprising a relatively low-voltage high-current winding, which is connected to rectifying means and then to a battery unit.
In a further related aspect the invention provides battery charging means as described above, wherein the control means is also capable of responding to the state of charge of the other battery units and modifying its response accordingly.
Preferably each supervisory means includes voltage and current measurement means and an algorithm for battery supervision which algorithm is relevant to the characteristics of the type of battery used.
Optionally, means for sensing failure or imminent failure of an identified battery unit may be employed to provide improved reliability.
Preferably the supervisory means corresponding to each battery unit is linked to other supervisory means by communications means.
Alternatively the battery bank may comprise just one battery and one unit of the invention.
In a further broad aspect the invention provides a method for maintenance of a bank of batteries, in which the method comprises the steps of separately charging up to a set level of charge, and separately discharging down to a set level of charge each monoblock or unit of the bank regardless of the amount of charge required to bring other monoblocks up to a set level of charge, so that the behaviour of any particular monoblock or unit has no effect on the remainder of the bank.